1. Field of the Invention
The present invention relates to a radio communication system, a radio communication method, a radio base station controller and a radio mobile station which are used in a mobile communication service and the like. More particularly, the present invention relates to a technique for keeping a communication even if a radio mobile station is moved in a radio communication system, namely, a hand-over technique.
2. Description of the Related Art
FIG. 1 shows an example of a radio communication system in which a plurality of base stations are connected to a mobile services switching center. As shown in FIG. 1, a territory (service area) to which a mobile communication network 200 gives a service is divided into radio zones 204 referred to as cells by a number of base stations (BS) 202 and covered by them. Several base stations 202 are bundled by a mobile services switching center (MSC) 206, and managed and controlled by each mobile services switching center 206. A mobile station (MS) 208 carries out a radio communication with any of many base stations 202, and switches a base station 202 of a communication partner, in conjunction with its movement. Also, a gateway mobile services switching center (G-MSC) 210 functions as a relay point when the mobile communication network 200 is mutually connected to another fixed network 212. The mobile communication network 200 is connected through the gateway mobile services switching center 210 to another fixed network 212.
Here, when the mobile station 208 crosses one cell 204 during a communication, the continuation of the communication with the base station 202 requires a switching of a communication line, namely, a hand-over process for changing the base station 202 connected to the mobile station 208. In the system shown in FIG. 1, the hand-over process is carried out in accordance with a control signal (for example, a message for a hand-over) outputted to the mobile services switching center 206 through the base station 202 during the connection from the mobile station 208.
However, if a radius of a cell covered by each base station 202 is small, a switching opportunity of a communication line is frequently done, which requires a very complex process. Especially, in a case of AHS (Advanced cruise-assist Highway System) remarked as one of Intelligent Transport Systems (ITS), a radius of a cell is very small such as 50 m to 100 m at most. Thus, the switching frequency of the communication line of the mobile station 208 becomes very high. In short, a period while a car moving at a high speed stays within one cell is very short so that the hand-over process must be frequently done. If the conventional hand-over technique is applied to such AHS, a rate of the hand-over process occupying the communication process becomes very large. This results in a problem that a communication efficiency becomes very poor.
Moreover, the AHS is a system for assisting an automatic run of a car. Thus, a high reliability is required of the AHS. Hence, the continuation of a communication at a time of a movement between cells is an essential function in the AHS. So, a failure of the hand-over is not permitted. However, the conventional hand-over technique does not insure the reservation of a communication line in a hand-over destination, namely, a movement destination. In short, if the communication lines at the hand-over destination are all used, a car during a communication can not carry out the hand-over. This results in a problem that the communication with the base station is interrupted.
As mentioned above, the frequency of the hand-over process becomes very high in the radio communication system, such as the AHS or the like, in which an area (cell) covered by one base station is narrow and a fast mobile body is targeted. Thus, the conventional hand-over technique has the problems of a deterioration in a communication efficiency and a low reliability of a communication.